https://gitlab.kitware.com/cmake/cmake/issues/18088 これが umbrella issue のようですね。 undefined_arch 問題にもリンクあり。

Xcode 10 Support (#18088) · Issues · CMake / CMake

Xcode 10 comes with a "new build system" for which CMake will need updates to keep things working. I'm opening this as a tracking issue. CMake's test suite has many...